The point of ActivityPub should be to become as widespread as possible so as to proliferate the standard and eliminate silos and walled gardens–including Facebook and its ilk. It would be an unambiguously good thing if Meta’s (and Tumblr’s) move towards interoperability cascades to the other big platforms like Instagram, YouTube, Twitter, and Reddit.

You could follow (or choose not to follow) users on any platform you want, from any platform you want.